The present invention is directed generally to an electroplating tank and, in particular, to an improved electroplating tank for electroplating items of jewelry or the like with gold or other metals on a rack fixture with improved uniformity of plating thickness distribution over items on the rack.
The process of electroplating of jewelry is generally performed utilizing a variety of electrolytic solution formulations in either rotating barrels or on rigid rack fixtures. The rack fixture method is used more commonly on delicate or highly polished items and constitutes a major portion of electroplating jewelry production. The electroplating process is utilized throughout the decorative jewelry industry to deposit layers of copper, nickel, gold, rhodium, silver and like metals onto jewelry items to provide an aesthetic appearance, and corrosion wear resistance characteristics, thereby insuring and enhancing the quality, appearance and value of the products.
In a conventional electroplating tank, the items of jewelry to be plated are wired onto a rack fixture which forms the negative terminal or cathode of the tank. Multiple rows and columns of jewelry are strung on the rack. An electrical field is created in the chemical plating solution between the rack which forms the cathode, and positive terminals or anodes.
The electrolyte solution is rich in the ions of the metal to be plated. Some of the metal ions in the electrolyte solution adjacent the parts to be plated are deposited onto the items on the rack and are electrically neutralized. Prior art patents which describe conventional plating techniques include U.S. Pat. Nos. 4,115,213, 4,174,261, 4,385,967, 4,595,478 and 4,634,503.
In conventional tanks, rack-fixtured jewelry items have the inherent problem of non-uniformity in the thickness of the deposited metal from item to item on the rack. This is due primarily to the non-uniform electrical field created between the positive terminals or anodes and the negative terminal or cathode when an electrical direct current (DC) is directed through the electroplating chemical solution. The field strength of the electrical field is higher on the edges of the rack as compared to the strength of the field at the central areas of the rack, thereby resulting in a lower metal plating thickness at the center of the rack and a higher metal plating thickness at the rack edges.
Such non-uniformity in electrical fields is directly apparent in the non-uniformity of jewelry items plated in a conventional system. Apart from the quality problems inherent in non-uniform coating procedures, where gold or other expensive metals are to be plated, substantial increased costs are realized through the wasting of the gold. In other words, since sufficient gold must be deposited on all items on the rack, the jewelry pieces at the rack edges receive a plating of more gold than is necessary for quality control standards in order to insure that jewelry items in the center of the rack are sufficiently plated.
It has been found that conventional gold electroplating tanks often suffer from a thickness distribution variance of more than about 11.5%. If this percentage could be reduced even slightly, significant cost savings could be realized immediately.
Accordingly, it is desired to provide an improved electroplating tank which overcomes the problems inherent in prior art constructions which provides a more uniform distribution of deposited metal from item to item on the plating rack.